1. Field of the Invention
The present invention relates to a process unit positioning device which performs mutual positioning of process units individually including photosensitive drums corresponding to different colors of toner, and an image forming apparatus which includes the process unit positioning device and forms a full-color image.
2. Description of Related Art
Up to now, different kinds of so-called four-tandem structure are proposed as an image forming apparatus which uses respective colors of toner, that is, yellow, cyan, magenta, and black, to form a full-color image by electrophotographic process. The image forming apparatus of this kind has a structure for obtaining a full-color image by arranging four photosensitive drums corresponding to the above-mentioned respective colors in parallel and superimposing toner images, which are obtained by developing electrostatic latent images on the respective photosensitive drums with the respective colors of toner, successively on an intermediate transfer member or a paper sheet.
The four-tandem image forming apparatus as described above needs to maintain high mutual positioning accuracy for process units individually including the respective photosensitive drums in order to prevent the toner images in the respective colors from being displaced from one another on the intermediate transfer member or the paper sheet. In a normal state, drum shafts of the respective photosensitive drums are supported by a positioning plate having bearing holes into which the respective drum shafts are fitted, which enables positioning of the respective process units with high accuracy.
Incidentally, the respective process units need to be replaced by a user because of machine life (for example, life of photosensitive drums) when the number of sheets printed reaches to a predetermined number (for example, tens of thousands of sheets). Therefore, in order to replace the respective process units, the positioning plate needs to be structured to be detachable/attachable from/to an apparatus main body.
For example, in some image forming apparatuses, a handle is formed on the positioning plate, and the user can grip the handle to fit the positioning plate onto the respective drum shafts (to fit the drum shafts into the bearing holes of the positioning plate) and to draw out the positioning plate from the respective drum shafts (to release the fit of the drum shafts into the bearing holes). The positioning plate drawn out from the respective drum shafts is placed on a different site outside the apparatus main body, thereby being completely separated from the apparatus main body.
However, in the case where the positioning plate is completely separated from the apparatus main body after the positioning plate is drawn out from the respective drum shafts, the user needs to locate a fitting position of the positioning plate with a sense of his/her hand in order to fit the positioning plate onto the respective drum shafts after the replacement of the respective process units, which inhibits the positioning plate from being fitted smoothly. This reduces workability in replacement of the respective process units. Further, the positioning plate is placed outside the apparatus main body after the positioning plate is drawn out from the respective drum shafts, and hence a moving distance for mounting/removal of the positioning plate to/from the apparatus main body is long, which further reduces the above-mentioned workability.
Further, in the case of mounting the positioning plate to the apparatus main body, when a fitting direction of the drum shafts toward the bearing holes does not coincide with a drum shaft direction because, for example, the fitting direction is inclined toward the drum shaft direction, the drum shafts become hard to be fitted into the bearing holes due to increased frictional resistance and stress caused upon contact.
Further, in the case of replacing the process units, developing rollers arranged in abutment with or in proximity to the respective photosensitive drums need to be retracted from the photosensitive drums. For example, a developing device is disclosed which is provided with a roller separating/contacting mechanism for bringing the developing roller into contact with the photosensitive drum in accordance with an action of attaching the developing device to an image forming apparatus main body or a closing action of an open/close unit while retracting the developing roller from the photosensitive drum in accordance with an action of removing the developing device or an opening action of the open/close unit. More specifically, in the above-mentioned roller separating/contacting mechanism, a shaft member for separating/bringing the developing roller from/into contact with the photosensitive drum is provided so as to protrude from a front surface of a developing unit, and the shaft member is pushed by the positioning plate to thereby bring the developing roller into proximity to the photosensitive drum while the positioning plate is opened to thereby retract the developing roller from the photosensitive drum.
However, this structure necessitates a certain amount of force to attach the positioning plate due to a reaction force acted upon the shaft member by the attachment of the positioning plate, which inhibits attachment work from being performed smoothly.